The other day I noticed my Xmas tree makes a high pitched whistling sound when it's turned on.

I've noticed this in other gadgets, too, but never in something so simple (two chips a ceramic capacitor and a couple of resistors). Does anybody know what causes them? Surely a sound requires a mechanical vibration....

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Many systems are driven by a "black box" which contains a switched-mode power supply. This uses a ferrite transformer (they tend to emit "mechanical" noise) and you are probably hearing either the frequency of oscillation or a sub-harmonic of it. Line transformers on early TV sets were notorious for the circa 20kHz whistle they produced, but only kids, cats and dogs were bothered by it.

In your case it could be the ceramic resonators that you are hearing (resonator being the key word)

It's actually amazingly simply to create sound from electrical "voltage" oscillations. As a youngster I "discovered" that by using a neon type screwdriver with the 'live" end stuck into a 240v live socket, and the "finger" end place into the ear using a steel knitting needle as a bridge-piece, one could actually hear the 50hz hum. Of course that was in the days when you were able to experiment without the safety elves ensuring that we all ended up frightened to live. I lived to tell the tale.

Most common-or-garden ceramic capacitors are barium-titanate formulations, which is a perfectly decent piezo electric materialand ferro-electric - ferro electrics are automatically microphonic and have high dielectric constants - hence their use in high value ceramic capacitors and piezo elements.

Such capacitors are used where the precise value and linearity is not important - ie decoupling. Not agood choice for an audio amplifier though.

Lower value ceramics can be non-ferro-electric and much more temperature-stable - there are many ceramic dielectric materials tochoose from

[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Speaking of a screwdriver, you might try actually localizing the source of the sound byholding a flat screwdriver tip against the various components, and then placing yourear against the handle, as a sort of mechanical-to-ear transducer. This works on autoengines for listening to the sound of valve tappets, for example.